Energy storage systems using large superconducting magnets have been proposed for leveling daily load requirements on electrical utility systems. Excess energy generated during off-peak hours can be stored and later returned to the power grid during high demand periods. By connecting the superconducting energy storage magnet to the power system with a bridge-type inverter, it is possible to obtain very efficient energy transfer between the storage magnet and the power system, as more fully described in U.S. Pat. No. 4,122,512 to Peterson, et al.
The large energy storage magnets proposed for storing sufficient energy to allow load leveling on a power grid utilize multiple turns of composite conductor formed of normal and superconducting material. The current flowing in the turns of the magnet naturally produces a net magnetic field, and any conductor in the field will experience a force at each point on the conductor oriented at right angles to the current and the magnetic field. Since superconducting magnets of the size proposed for electrical system energy storage will conduct extremely large currents and will generate strong magnetic fields, the forces experienced by the conductors will be very large. Since no conductor by itself could possibly withstand the forces that would be exerted on the conductor under these conditions, an external support structure capable of resisting the large loads imposed on the conductor is thus necessary.
However, substantial practical difficulties are encountered in supporting the superconducting magnet because of the supercooled conditions under which the magnets must be operated. For example, the support structure must not add a significant thermal load on the cooling system. Generally, the magnet is coupled to a warm structure, such as bedrock, by struts. The bedrock is at normal ambient temperatures (e.g., 50.degree. to 70.degree. F.). The magnet, however, is operated in a Dewar which thermally isolates the magnet for operation at cryogenic temperatures (e.g., 1.8-7K). Although the Dewar which encases the magnet provides some thermal insulation, significant heat transfer from the magnet to the warm structure occurs through the struts, which must pass through the walls of the Dewar. These struts are often the major source of refrigeration load for the system. Although it is desirable that the struts minimize heat conductivity, they must be strong enough to provide adequate structural support for the system and to transfer the very large magnetic forces from the magnet to the warm structure. In addition, struts are also needed to support the weight of the magnet.
High strength cryogenic strut designs have generally reinforced the strut in the axial direction, commonly with fibers oriented in or close to the axial direction. Reinforcement of the struts in this manner leads to increased thermal conductivity in the axial direction since the fibers extend in substantially a direct line, and thus provide a short thermal resistance path, from the cold magnet to the warm support structure. The thermal conductivity of reinforcement fibers is generally higher than that of the matrix resin.
One approach to the problem of adequately supporting a superconducting energy storage magnet is shown in U.S. Pat. No. 5,115,219 to Withers et al. This system relies on an adjustable external strut system and a vertical support system which accommodates the large radial movements of the magnet. The struts have a tubular body coupled to an insulating disk which retards heat transfer. The strut has a radial linear end with a ball which nests in a socket secured to a vertical interface plate.
A further approach is described in U.S. Pat. No. 3,980,981 issued to Boom et al. on Sep. 14, 1976. The struts or support columns include support members which criss-cross and intersect to provide stability. Further, the columns include lateral members between adjacent columns for providing structural stability. The columns are made from an epoxy-fiberglass composite.
Efforts to improve designs for a thermally insulated load bearing strut for use in such applications as superconductive magnetic energy storage (SMES) systems, superconducting supercollider (SSC) systems, and cryogenic storage Dewars have generally followed one of three approaches. The first is to design the struts with new materials which have superior support strength and superior thermal resistance. The second involves intercepting and removing the heat from the struts more efficiently. The third involves innovative but complex support designs.
The first approach generally relies on nonmetallic composites. Research on new composites with higher strength to thermal conductivity ratios has been in progress for a long time. An example is the Altex (TM) alumina fiber reinforced epoxy manufactured by Sumitomo Chemicals in Japan. However, these new advanced composites are usually more expensive than the widely used glass fiber-epoxy struts.
The second approach--intercepting and removing heat more efficiently--often relies on increasing the length of the heat path. However, this is disadvantageous because structural deficiencies are introduced due to buckling constraints, and the increased length increases costs associated with the size of the warm structure and the struts.
The third approach--utilizing innovative designs--has resulted in designs which transfer the forces from cold to warm structure by tension straps. Tension straps have been successful for cold structure suspended inside warm structures, as in a storage Dewar. Another innovative design utilizes a re-entrant strut which includes an elaborate sequence of coaxial tension-compression members to increase the heat path length without increasing the overall strut length. Re-entrant struts are complex to manufacture and require more material. Because of manufacturing complexity, the re-entrant strut has been used only for small loads.